The present invention relates to integrated signalling mechanisms for ATM-like services in shared medium networks. A shared medium network is a network that contains at least one multiple access (MA) segment. Users of a MA segment share a common medium link to access the network.
Shared medium segments may be classified according to their topologies, for example, ring topology (eg. a LAN), bus topology and star topology. The present invention is mainly applicable to shared medium segments having a star topology, which means that end systems on the network which share the shared medium cannot communicate directly with each other. Instead all end systems in the MA segment communicate with a headend, which is essentially a switch. The headend is responsible for routing traffic from the end systems of the shared medium segment to other end systems in the segment or to other parts of the network (the wide area network) and for routing traffic from the wide area network to the end systems.
This type of segment is essentially asymmetric in the sense that end systems share a common medium uplink to communicate with the headend, while the headend has a dedicated downlink to communicate with all the end systems. Problems are created by the reception of the same ATM stream (containing all traffic directed to all end systems in the segment) by all the end systems in the segment and by all the end systems in the segment having to compete to establish connections or calls on the common medium to communicate with the headend. These problems are exacerbated when there are large numbers of users in a segment.
The ATM stream may carry data traffic, ATM signalling traffic (eg. information for setting up a connection or call on the network) and ATM management traffic (eg. network administration information, including registration of end users).
An example of a shared medium network with star topology is a network, as shown in FIG. 1. The network in FIG. 1 has one MA segment, or common medium beam (2).
In adapting ATM to shared medium segments, a number of problems have to be solved. These include, maximising the useable amount of the shared medium bandwidth for data traffic, identifying the source of traffic in order to control access to the shared medium and for billing purposes and reducing delays to end systems for access to the shared medium. In addition where the headend comprises a geostationary satellite (4) supported by a ground based network controller (6), as shown in FIG. 1, propagation delay effects have to be minimised, for example, by reducing the number of messages exchanged to set up a call. Also, when the headend is a satellite a large number of end system have to be catered for on the common medium.
Furthermore, standard ATM virtual channels (VCs) which may have to be modified in order to transmit them over the shared medium segment, must appear to be standard ATM VCs to the wide area network and the end systems, if the shared medium segment is to be able to operate as part of a wide area network which uses conventional end system processing. Management VCs originating and terminating within the shared medium segment may be modified within the ATM Adaptation layers, but must be supported in an unmodified manner above this layer. In particular, the SNMP (Simple Network Management Protocol) layer of the ILMI (Interim Local Management Interface) must be unmodified. Also, signalling VCs originating and terminating within the shared medium may be modified within the ATM Adaptation layers but must be supported in an unmodified manner above this layer. In particular the call signalling layer (eg. Q.2931, PNNI, B-ISUP, BICI, AINI) should remain unchanged, ie. the SAAL (Signalling ATM Adaptation Layer) must provide a conventional SAAL service.
In a geostationary satellite system, round trip delay (ie. propagation delay for messages sent from an end system to the satellite and for response from the satellite to the end system) is high. Wireless telephony solutions such as GSM (Global System Mobile) tend to require multiple exchanges between an end system and an intermediate system in order to establish a call and so can generate unacceptable delays.
Conventional wireline ATM systems are known which operate in a point to point manner whereby a dedicated two-way simultaneous link exists between a port on an end system and a port on an intermediate system. The intermediate system performs four basic functions; cell relay, ATM end system registration, ATM peer intermediate system registration and ATM signalling. In a shared medium segment the activities of the intermediate system will have to be dealt with by the headend of the segment in combination with a NCC (Network Control Centre). For a network segment with a satellite headend the NCC will generally be ground based and thus physically separated from the headend.
The cell relay function consists of receiving a cell, examining the cell header, forwarding the cell to the appropriate output port and replacing the cell header with a new cell header.
The ATM end system registration function consists of allocating a unique address to each end system and associating the address with the appropriate intermediate system port. This is done by concatenating the end system""s unique address with the intermediate system""s assigned address. An automatic process using the SNMP of the ILMI conventionally does this over a standard VC identified by Virtual Path Identifier VPI=0 and Virtual Channel Identifier VCI=16 during the set up of the link between the port of the intermediate system and the end system.
The ATM peer intermediate system registration function consists of exchanging routing information with peer intermediate systems and associating routing information with each link to another intermediate system.
The ATM signalling function consists of exchanges between end systems and peer intermediate systems to establish call routing information which is subsequently used by the cell relay function. The system relies on having individual duplex signalling and ILMI channels to each end system. The ATM signalling protocol is run over a standard VC identified by VPI:VCI (0:5). The ATM signalling function also relies on the Service Specific Connection Orientated Protocol (SSCOP) to provide assured data delivery between peer signalling entities, eg. end systems and intermediate systems or peer intermediate systems.
SSCOP provides reliable transport of ATM signalling messages between two signalling entities using timeouts and retransmissions. The sender periodically sends a POLL Protocol Data Unit (PDU) to enquire about the state of the receiver. The receiver replies with a STAT PDU to tell the sender which packets were correctly received. The sender then uses the STAT messages to adjust its window size and retransmit lost packets. Once an SSCOP connection is established between two signalling entities it is kept alive by exchanging xe2x80x98keep alivexe2x80x99 messages periodically.
Conventional wireline ATM systems use the physical port on the intermediate system to identify the end system at the interface between a single end system and its associated intermediate system.
If multiple users are required per port an ATM Forum UNI 4.0 can be used to support the multiple users using a virtual UNI concept. The virtual UNI concept assigns each end user one or more Virtual Path Connections (VPCs) using the VPI addressing field and the UNI uses a VP cross-connect to combine the user VPCs so that the intermediate system effectively sees a single end system. As the VPI addressing space comprises a single octet, there is a limit of 256 end systems per port.
An improvement of the Virtual UNI scheme is possible where the 24 bits of VPI (8 bits) and VCI (16 bits) are re-partitioned to give more to the VPI. For example an even division giving the VPI 12 bits could support 4000 end systems with 4000 VCs. It may happen that the 24 bit VCI/VPI space is already reduced to fewer bits by some systems that wish, for example, to use part of the space as an internal switch routing label.
Local Area Network (LAN) systems address every Protocol Data Unit (PDU) with a source address and a destination address. This enables logical point to point channels to be established over the LAN. The PDUs used in LANs are typically large with respect to the size of the addressing information. This is not the case in ATM networks in which addresses will be larger because of the larger number of end systems and the PDUs are smaller.
The end systems sending traffic to a headend via a common medium segment have to have their access to the common medium controlled to avoid interference between cells from different end systems. There are two fundamental methods of doing this medium access control (MAC) which are in common use. One is contention or random access in which end systems send call set up requests to the headend on a random basis. If the request is received by the headend, the headend informs the end system accordingly by sending an acknowledgement message and allocates the end system slots (eg. time-slot/frequency-slot pairs in a MF-TDMA uplink) on the common medium uplink on which it can send traffic. If the request is not received by the headend the end system waits and then tries again to send a call set up request. The other is controlled access in which each end system is allocated slots in the common medium uplink on which it can transmit a call set up request. This guarantees the end system a dedicated slot to set up a call. However, this takes up resource (ie. slots) from the common medium uplink which may not always be utilised by the end system and the dedicated slots may be separated in time such that delays are added to call set up because the end system has to wait for the dedicated slot in order to transmit a call set up request.
In a MF-TDMA scheme, the time slots allocated to an end system by the intermediate system could be in different frequencies and may not be adjacent. Each time slot carries one ATM cell. The MF-TDMA signal that carries ATM cells from all the end systems in a multiple access link is demultiplexed into a single ATM stream that is delivered to a port in the intermediate system. Due to the access mechanism ATM cells that were transmitted consecutively by an end system may arrive at the port in the intermediate system interleaved with ATM cells that were sent by another end system. The intermediate system processes all incoming cells consecutively. It checks the VPI/VCI value in each ATM cell and puts all the cells with the same VPI/VCI in the same queue to be reassembled. In conventional ATM signalling many end systems on the same shared medium use the same VPI/VCI value for signalling messages and at least some of these signalling messages may need to be segmented into a number of ATM cells. A signalling message generated by a first end system is segmented into a number of ATM cells identified by a VPI/VCI. These ATM cells are then transmitted using the time slots allocated to the first end system. A signalling message generated by a second end system is also segmented into a number of ATM cells identified by the same VPI/VCI. These ATM signalling cells are then transmitted using the time slots allocated to the second end system. At the receiving port of the intermediate system, the time slots allocated to the first end system may be interleaved with the time slots allocated to the second end system, and so the intermediate system will not be able to distinguish between ATM signalling cells from the first and second end systems and so will be unable to reassemble the signalling messages from the two end systems correctly. Therefore, it is necessary that the ATM cells from the different end systems are distinguishable from each other in some way, or that each signalling message fits into a single ATM cell.
A conventional signalling adaptation layer structure or SAAL of an end systems (8,10) and an intermediate system (4,6) is shown in FIG. 6. The SSCOP layer provides flow control and error recovery. In normal operation peer SSCOP entities (eg. in an end system and an intermediate system) exchange keep alive messages. This is used to detect link failure. The SSCOP state, ie. whether the SSCOP is up or down, is conventionally propagated to the signalling layer Q.2931 as an indication of the state of the SAAL and is used to control the behaviour of the signalling layer. The SSCOP state is in turn dependent on the state of the ATM layer and the physical layer.
In a conventional point to point wireline system there is a dedicated physical link (the physical layer) between the end system and the intermediate system. Part of the capacity of this link is dedicated to ATM signalling traffic. As long as the physical link is available, an SSCOP connection can be kept alive by sending messages between peer SSCOPs at regular intervals, so that a call can be established at any time without the need to bring up a new SSCOP connection. However, in a shared medium uplink, because the link between the end systems and the intermediate system are shared by all the end systems the capacity of the uplink is scarce and the keep alive messages will take up a some of this capacity, reducing capacity on the uplink for other traffic.
The object of the present invention is to provide integrated signalling mechanisms for supporting an ATM-Like shared medium network which overcome at least some of the problems discussed above.
According to a first aspect of the present invention there is provided a multiple access asynchronous network system for providing network access to a plurality of end systems over a shared medium uplink to a satellite headend supported by a network controller, wherein the network controller comprises means for allocating part of the uplink resource and the satellite headend comprises means for allocating part of the uplink resource on a temporary basis in response to end systems making a new request for uplink resource.
According to a second aspect of the present invention there is provided a method for providing network access to a plurality of end systems over a shared medium uplink of a multiple access asynchronous network segment to a satellite headend, which satellite headend is supported by a network controller, comprising the steps of temporarily allocating uplink resource to an end system using the satellite headend in response to said end system making a new request for uplink resource and subsequently allocating uplink resource to said end system using the network controller.
The part of the uplink resource allocated to the satellite may be one or more channels of the shared medium uplink or one or more slots from the shared medium uplink. When a new request for uplink resource is made by an end system the satellite will, if it has resource available, allocate resource immediately on receiving the request on a temporary basis to allow the end system to start its call before the network controller allocates resource on the uplink permanently. It will take the network controller longer than the satellite to allocate resource because the new request for resource and the allocation of resource from the network controller have to be communicated between the end system and the network controller via the satellite.
According to a third aspect of the present invention there is provided a signalling system for use in a multiple access asynchronous network segment for providing network access to a plurality of end systems over a shared medium uplink to an intermediate system wherein the intermediate system comprises means for allocating an address to each end system and means for inserting the end system address into the VPI/VCI space of a cell which is to be transmitted over the segment and which is associated with that end system using substantially all the VPI/VCI space of the cell. By using all of the VPI/VCI space available in a cell, which is usually 24 bits, to contain end system addresses, the number of end systems which can be addressed is vastly increased and so the segment is able to support more end systems.
Preferably, each end system comprises means for inserting the end system address into the VPI/VCI space of a cell which is associated with that end system and which is to be transmitted over the segment using substantially all the VPI/VCI space of the cell.
Preferably, the intermediate system allocates each end system a signalling address (i.e. signalling VPI/VCI), a management address (i.e. ILMI VPI/VCI) and/or a data address (i.e.connection VPI/VCI). Thus, a cell containing signalling information associated with an end system will contain that end system""s signalling address also referred to herein as an ATM signalling virtual channel indicator, in the VPI/VCI space of the cell. If the cell is transmitted from the end system to the intermediate system on the shared medium uplink, the intermediate system will recognise it as a cell from that end system which contains signalling information. Similarly, a cell containing management (e.g ILMI) information associated with an end system will contain that end system""s management address, also referred to herein as an Interim Local Management Interface (ILMI) virtual channel indicator, in the VPI/VCI space of the cell and a cell containing data associated with an end system will contain that end system""s data address. This enables the intermediate system to identify the end system from which the cells originate and the type of data carried in the cell.
The intermediate system can allocate the addresses either statically or dynamically to end systems. Dynamic allocation is preferred because it increases the number of end users that can be accommodated on the uplink because, at any one time, only these end systems accessing the segment have an address allocated on them.
A broadcast downlink will generally supplement the shared medium uplink in the network segment and it is preferred that an address in the VPI/VCI space of a cell sent over the downlink to a particular end system is the same as the address in the VPI/VCI space of a cell sent on the uplink from that end system. This enables the end system to identify cells on the broadcast downlink that are intended for it by checking the VPI/VCI space of all cells transmitted on the downlink. Those cells that do not contain an address of that end system will be discarded by that end system.
According to a forth aspect of the present invention there is provided a signalling method for use in a multiple access asynchronous network segment for providing network access for a plurality of end systems to an intermediate system over a shared medium uplink, in which substantially the entire VPI/VCI space of a cell transmitted over the segment is made available to contain an address of an end system associated with that cell.
Dynamic allocation of end system addresses using the VPI/VCI space, is preferred because end system addresses will only be required for the proportion of end systems using the MA segment at any one time. When end system addresses are allocated permanently (ie. static allocation) then all the end systems which can access the MA segment require dedicated addresses.
Preferably, for point-to-multipoint connections when there are a plurality of receiving end systems in the same multiple access asynchronous network segment an intermediate system dynamically allocates the same end system address to all said plurality of receiving end systems. In this way, a single copy of each cell can be relayed on the downlink of each such segment and all the receiving end systems on that segment will receive it. This reduces traffic on the broadcast downlink, which is a scarce resource.
Where the allocation of a signalling address, also referred to as an ATM signalling virtual channel indicator (VCI), to each end system is dynamic, preferably the end systems and intermediate system comprise a signalling layer and an SSCOP layer arranged so that an SSCOP connection between an end system and the intermediate system can be broken without effecting the operation of the signalling layer.
According to a fifth aspect of the present invention there is provided an Integrated Addressing and packet Segmentation And Reassembly (IASAR) system for use in a multiple access asynchronous network segment for providing network access for a plurality of end systems to an intermediate system over a shared medium, wherein the end systems and the intermediate system comprise means for inserting a Multiplexing Identifier (MID) address header designating an end system into every ATM cell generated from the segmentation of a packet. The end systems and the intermediate system preferably also comprise means for extracting an MID address header designating an end system from each received ATM cell before reassembling the packet. The packet may contain a signalling or management message.
According to a sixth aspect of the present invention there is provided a method for Integrated Addressing and packet Segmentation And Reassembly (IASAR) system for use in a multiple access asynchronous network segment for providing network access for a plurality of end systems to an intermediate system over a shared medium, comprising the steps of segmenting a packet to generate a group of cells and inserting a MID address header designating an end system into each cell of this group. Preferably, the method additionally comprises the steps of extracting a MID address header designating an end system from each received cell and reassembling the packet from a group of cells with the same MID.
In a preferred embodiment of the fifth and sixth aspects of the present invention, a multiplicity of signalling channels each associated by a MID address header with an individual end system are carried over a single virtual channel (VC). The multiplicity of signalling channels correspond to the multiplicity of end systems. By using the MID address header to identify the end system, a single signalling VC, can then be shared by a multiplicity of end systems. Effectively, the MID address header multiplexes a plurality of signalling channels onto a single virtual channel.
Similarly, a multiplicity of ILMI management channels each associated by the MID address header with an individual end system can be carried over a single virtual channel (VC).
In a preferred embodiment of the fifth and sixth aspects of the present invention the ATM Adaptation Layer for the signalling and ILMI stack of an end system and/or intermediate system SAAL comprises an AAL xc2xe ATM Adaptation layer including a Packet Segmentation and Reassembly sub-layer for generating ATM cells which include a multi-bit field for the MID address header. The AAL xc2xe ATM adaptation layer may comprise a Service Specific Convergence Sublayer (SSCS), which in the preferred embodiment of the present invention is the SSCOP layer, a Common Part Convergence Sublayer (CPCS), as well as the Segmentation and Reassembly (SAR) Sublayer. SAR Protocol Data Units of AAL xc2xe include a 10-bit field for the Multiplexing Identifier header. Therefore AAL xc2xe allows the multiplexing of a maximum of 1024 signalling or management connections in the same VPI/VCI.
According to a seventh aspect of the present invention there is provided an intermediate system for use in a multiple access asynchronous network segment for providing network access for a plurality of end systems to an intermediate system over a shared medium uplink, wherein the intermediate system comprises means for receiving a meta-signalling message from an end system containing the MAC address of that end system, means for allocating a signalling VC and/or a management VC and a Multiplexing Identifier (MID) value to that end system and means for transmitting a meta-signalling message to the end systems in the network segment containing the MAC address of that end system and the allocated VC(s) and MID value.
According to an eighth aspect of the present invention there is provided a signalling method for use in a multiple access asynchronous network segment for providing network access for a plurality of end systems to an intermediate system over a shared medium uplink, in which the intermediate system allocates a signalling VC and/or management VC and a Multiplexing Identifier (MID) value by means of an exchange of meta-signalling messages between the end system and the intermediate system which meta-signalling messages contain the end systems MAC address.
Meta-signalling messages are 48 bytes long, so that they can fit in the payload of a single ATM cell. ATM cells containing meta-signalling messages are identified by a well-known VPI/VCI. All the end systems in a shared uplink use this VPI/CI for sending meta-signalling messages.
In a preferred implementation of the seventh and eight aspect of the present invention, the VPI value used for meta-signalling could be associated with the beam number in order to facilitate the routing of meta-signalling messages to the destination beam.
The MID values will be used by the AAL layer of the end systems and the intermediate systems to multiplex multiple signalling and/or management connections in the same VPI/VCI in the multiple access segment. The intermediate system uses the MAC address contained in the meta-signalling message sent by the end system to identify the end system that has requested the signalling VC and/or management VC. Each end system uses the MAC address contained in the meta-signalling message sent by the intermediate system to identify whether the allocation or denial of a signalling and/or management VC is for it.
According to the ninth aspect of the present invention there is provided a signalling method for use in a multiple access asynchronous network segment for providing network access for a plurality of end systems to an intermediate system over a shared medium uplink, comprising the steps of:
an end system generating a meta-signalling message containing a request for a signalling and/or a management VC and the MAC address of that end system and encapsulating the meta-signalling message in an ATM cell, identified by a predetermined VPI/VCI as containing meta-signalling information,
the intermediate system processing all ATM cells with a meta-signalling VPI/VCI by;
either allocating a signalling and/or a management VC and a MID address to the end system, inserting the allocation and the MAC address of the end system in a xe2x80x9creplyxe2x80x9d meta-signalling message and encapsulating the xe2x80x9creplyxe2x80x9d message in an ATM cell with a meta-signalling VPI/VCI and transmitting it over the downlink,
or generating a xe2x80x9cdeniedxe2x80x9d meta-signalling message including the MAC address of the end system and encapsulating the xe2x80x9cdeniedxe2x80x9d message in an ATM cell with a meta-signalling VPI/VCI and transmitting it over the downlink, wherein
each end system receives and processes all ATM cells with a meta-signalling VPI/VCI by examining the MAC address contained in the meta-signalling message and if the MAC address is the MAC address of that end system retains the allocation for further processing or otherwise discards the allocation.
According to a tenth aspect of the present invention there is provided a signalling system for use in a multiple access asynchronous network segment for providing network access for a plurality of end systems to an intermediate system over a shared medium uplink wherein each end system and the intermediate system comprise a signalling layer and an SSCOP layer arranged so that SSCOP connections between each end system and the intermediate system can be broken without effecting the operation of the signalling layers. Preferably, the signalling layer in the end system comprises means for initiating sending of a message by joining together an SSCOP establishment message and a signalling call set up message to form a composite request message which is transmitted over the common medium uplink.
When the segment also comprises a broadcast downlink, it is preferred that the a signalling layer in the intermediate system comprises means for initiating sending of a message to one of the end systems, by joining together an SSCOP establishment message and a signalling call set up message to form a composite message which is transmitted over the common medium downlink.
According to an eleventh aspect of the present invention there is provided a signalling method for use in a multiple access asynchronous network segment for providing network access for a plurality of end systems to an intermediate system over a shared medium uplink wherein each end system and the intermediate system comprise a signalling layer and an SSCOP layer, comprising the steps of breaking SSCOP connections between each end system and the intermediate system without effecting the operation of the signalling layers. Preferably the method additionally comprises the steps of initiating sending of a message in an end system by joining together an SSCOP establishment message and a signalling call set up message to form a composite request message and transmitting said composite request message over the common medium uplink.
Where the segment comprises a broadcast downlink, the method preferably comprises the steps of initiating sending of a message in the intermediate system to one of the end systems by joining together an SSCOP establishment message and a signalling call set up message to form a composite message and transmitting said composite message over the common medium downlink.
In all of the abovementioned aspects of the present invention in a preferred embodiment the intermediate system comprises a satellite headend supported by a network controller. The network controller will generally be ground based.